The invention relates to a display device comprising a display window and means for generating light at areas of the display window and means for selectively time-sequentially activating the said areas, said display device comprising positioned in front of the display window, or forming part of the display window, a means for selectively transmitting light from activated areas, and blocking at least partly light from non-activated areas.
Many display devices of many different types are known, for instance CRT""s (Cathode ray tubes), PDP (Plasma Display Devices), LCD (Liquid Crystal Displays), and many other types.
In general a display device comprises a display window. The image is displayed on the display window. The display window comprises means to selectively generate light at areas of the display window. In a CRT for instance the image is built line-by-line.
A major problem for display devices is formed by reflection of ambient light on the display window or at components of the display device such as phosphor elements (in e.g. CRT""s and PDP""s). The viewer sees, apart from the image generated by the device, also reflections of other light sources, such as lamps and/or the sun shining on the display window. The reflections of such external (i.e. outside the display device) light sources reduces the contrast of the image displayed, and can even make it invisible especially when bright sunlight shines on the display window. Many solutions have been proposed ranging from reducing the light intensity in the room, reducing the reflection coefficient of the surfaces of the display window (anti-reflection coatings) and using dark glass for the window (the latter reduces the reflection on the inside of the display window).
It is known from European patent application no. 0 000 422 to use a means for transmitting light from activated areas, and blocking at least partly light from non-activated areas. Areas that are activated (i.e. emit light) are then seen, whereas non-active areas are black. Such a means can strongly increase the contrast. The means shuts out light from non-activated areas and transmits light from activated areas. The intensity of the image itself is not, or only to a small amount reduced, whereas the intensity of the reflected light can be strongly reduced.
Although this idea has been known for some time, and seems to be a simple and elegant solution to the problem, as yet it has not been possible to implement this solution in practice.
The gain in contrast is in practice not enough to justify the costs. It is an object of the invention to provide a display device of the type described in the opening paragraph having an increased performance.
To this end the means for transmitting light from activated areas, and blocking at least partly light from non-activated areas comprise an LCD cell or LCD cells, the LCD cell(s) comprising an oriented polymer network, an LC material with negative xcex94xcex5 and a pleochroic dye, dispersed in the oriented matrix, the passive relaxation time being less than 2 milliseconds.
The invention is based on the insight that the LC cells have to xe2x80x98open and closexe2x80x99 fast, preferably within 2 milliseconds to be truly effective. An LCD cell is switched from one state (transmissive or non-transmissive) to another (non-transmissive or transmissive) in two different fashions, actively by applying voltages, and passively when the voltage over the cell is removed. By using an oriented polymer matrix and an LC material with negative xcex94xcex5 a fast passive relaxation (measured as the half-way point between an open and shut orientation) when the voltage is removed of smaller than 2 milliseconds is possible.
The cell comprises also a pleochroic dye. An LCD cell with an oriented polymer network and an LC material with negative xcex94xcex5 has in the non-transmissive state a high to very high scattering coefficient which would increase the reflection of outside light sources, rather than decrease it, leading to a decrease in contrast. However, the pleochroic dye absorbs light inside the LCD cell, reducing the scattered light intensity.
The presence of the pleochroic dye also decreases the effective passive relaxation time.
Preferably the cell thickness lies between 6 and 25 micrometer.
Preferably the content of the oriented polymer matrix lies between 5 and 15%. The more oriented polymer matrix the LCD cell comprises, the faster the passive relaxation is, but the more scattering occurs, also in the transmissive state. Below 5% it becomes difficult to obtain fast passive relaxation, above 15% scattering becomes a problem. The device may comprise a number of LCD cells, or a single LCD cell with many electrodes.